Two bugs fixed:
- data_received only split on CRLF, so clients sending bare LF (telnet, nc,
some libraries) got no responses at all. Now splits on LF and strips
trailing CR, matching real Postfix behavior.
- AUTH PLAIN without inline credentials set state to "await_plain" but no
handler existed for that state, causing the next line to be dispatched as
a normal command. Added the missing state handler.
